Radiation detector suitable for tympanic temperature measurement

ABSTRACT

Tympanic temperature measurements are obtained from the output of a thermopile mounted in an extension from a housing. The housing has a temperature display thereon and supports the electronics for responding to sensed radiation. A disposable plastic sheet taken from a tape of such sheets stretches over the end of the extension between side posts. The thermopile is mounted in a highly conductive thermal mass which includes a waveguide tube. A low conductivity gaseous environment surrounding the thermopile extends through the tube. The electronics include an EEPROM in which both calibration and characterization information may be stored during a calibration procedure through an optical coupling. A capacitor and transistor associated with a switch form a simple watchdog circuit.

The most common way of measuring a patient's temperature is by use of asublingual thermometer, that is, one placed under the tongue. Suchthermometers have suffered several disadvantages. Accuracy of a readingdepends on the mouth remaining closed and the thermometer being properlypositioned under the tongue. Drinking liquids or breathing through themouth prior to taking a measurement can affect the reading. Further, themouth is a source of mucous which presents a significant risk ofcross-contamination. Also, the cost per reading of such instruments hastypically been high.

One type of sublingual thermometer is the common mercury thermometer.Such thermometers have the disadvantage of taking a considerable amountof time to reach a steady state temperature in order to provide anaccurate reading. Further, they are easily broken, requireserialization, and are difficult to read.

As an alternative to the mercury thermometer, disposable liquid crystalthermometers are often favored. As a disposable item, the serializationrequirement is eliminated, but the cost per reading is high.

To decrease the time required to obtain a patient's temperature,electronic thermometers have been developed. Such thermometers typicallyinclude a thermistor which may be positioned in a disposable cover.Although the thermometers do not reach a steady state temperature duringtheir measurement time of 15 to 30 seconds, through electronicinterpolation a steady state temperature may be estimated from thetemperature readings throughout the 15 to 30 seconds. The thermometersare often cumbersome, and as with other sublingual thermometers thetemperature readings may be unreliable in certain circumstances,especially when the probe is not precisely placed under the tongue.

Another electronic temperature device is a tympanic temperaturemeasurement device. Such devices rely on a measurement of thetemperature of the tympanic membrane area in the ear by detection ofinfrared radiation. The tympanic membrane area is often considered to bemore representative of a patient's core temperature, and infraredtemperature measurements using a thermopile are extremely rapid.Disposable sleeves may be placed over the radiation detector. Acommercial tympanic temperature measurement device is illustrated inU.S. Pat. No. 4,602,642 to O'Hara et.al. As suggested in that patent,the infrared detection approach does present demands on theinstrumentation to avoid inaccuracies due to ambient temperature andspurious heat flux to the thermopile.

SUMMARY OF THE INVENTION

The present invention relates to various features of a radiationdetector which make the detector particularly suited to tympanictemperature measurements without certain deficiencies of prior tympanictemperature detectors. For example, the O'Hara et.al. system relies onheating of the radiation probe to a precise temperature to maintaincalibration of the device during a test. As a result, the instrument isnot usable where the ambient temperature exceeds that precisetemperature. Also, to assure proper calibration for each test, theO'Hara et.al. system uses a light chopper-type of calibration unithaving a target heated to approximately 98° F. Before each test, thethermopile in the probe is calibrated as it views the chopper unittarget. Once removed from the chopper, the temperature reading must beobtained promptly because the probe will cool after removal from theunit and thus introduce errors. This requirement for calibration in thechopper unit prior to each temperature reading imposes a rigid protocolon the user which is more cumbersome than that of electronicthermometers. Further, the requirement for heating the target and theprobe adds bulk and weight to the system. The present . inventionprovides for a radiation detector which is at all times properlycalibrated without heating of the thermopile and without a choppercalibration unit. As a result, the instrument is less cumbersome, usesless power and provides quicker readings without having to follow anextensive protocol.

In accordance with one aspect of the present invention, a thermopile ismounted within a thermal mass and has a junction thermally coupled tothe thermal mass. A thermally conductive, reflective tube is coupled tothe thermal mass for guiding radiation to the thermopile from anexternal target. A thermal barrier surrounds the thermal mass and tube.The temperature of the thermal mass, and thus of the thermopile coldjunction, is allowed to float with ambient. A temperature measurement ofthe thermal mass is made to compensate the thermopile output.

Temperature differences between the tube and thermopile cold junctionwould lead to inaccurate readings. To avoid those differences, the largethermal mass minimizes temperature changes from heat which passesthrough the thermal barrier, and good conductivity within the massincreases conductance and minimizes temperature gradients. The outerthermal RC time constant for thermal conduction through the thermalbarrier to the thermal mass and tube is at least two, and preferably atleast three, orders of magnitude greater than the inner thermal RC timeconstant for the temperature response of the cold junction to heattransferred to the tube and thermal mass. For prompt readings, the innerRC time constant should be about 1/2 second or less.

Preferably, the thermopile is mounted to a film suspended within a ring.The ring is supported on electrically conductive pins extended throughan adjacent ring to the side of the film on which the thermopile ismounted. The film is spaced from the adjacent ring, and the rings andthermopile are surrounded by a low conductivity gaseous volume.Preferably, the low conductivity gaseous volume extends through thelength of the conductive tube. The space between the film and theadjacent ring through which the conductors extend is filled withthermally conductive material.

The thermopile may be mounted in a can which encloses the lowconductivity gaseous volume. The thermal mass may comprise an annularmember which surrounds the can and a length of the tube adjacent to thecan. The annular member is tapered about its outer periphery toward thetube. A conductive plug is positioned behind the can within the annularmember. The can, tube, annular member and plug are bonded together byhigh thermal conductivity material such as solder, epoxy, or powderedmetal to obtain a continuous low resistance path from the end of thetube to the cold junction of the thermopile. Alternatively, the partsmay be press fit together to provide the high conductance bond. Thethermal barrier comprises a sleeve spaced from the thermal mass andtube. The sleeve is tapered toward the end of the tube away from thecan.

Preferably, a probe extension which supports the radiation sensorextends from a housing which displays the tympanic temperature. Thishousing supports battery powered electronics for converting radiationsensed by the sensor to tympanic temperature displayed by the display.The entire instrument may be housed in a single hand-held packagebecause a chopper calibration unit is not required. The small additionalweight of the electronics in the hand-held unit is acceptable becausereadings can be made quickly. The readings can be made in less than fiveseconds, and preferably in less than two seconds.

Preferably, the probe extension extends about orthogonally from anintermediate extension which extends at an angle of about 15° from anend of the housing. The surface of the extension curves outwardly alongits length from its distal end following a curve similar to that of anotoscope. A sanitary cover in the form of a removable plastic sheet maybe stretched over the end of the probe. The sheet may be retained on theprobe by posts on the sides of the probe over which holes in the sheetare positioned.

Many of the sheets can be formed in a tape of transparent, flexiblemembrane segmented into individual covers by frangible lengths acrossthe tape. The holes adapted to retain the sheet across the probe areformed to each side of each frangible length. Reinforcement tape may bepositioned on the tape, and the frangible lengths may be formed as byperforations through the reinforcement tapes. In the presentapplication, the membrane must be transparent to infrared radiation. Thecovers may be adapted to other measuring instruments by using membraneswhich are transparent, for example, to visual light, sound or the like.Polyethylene sheet is preferred for infrared measurements.

The electronics may include an optical signal detector for receiving adigital input, and an electrically erasable programmable read onlymemory (EEPROM). A processor is programmed to respond to input from theoptical signal detector to store information in the EEPROM and to usethe stored information to respond to radiation and to drive the display.The processor may also be programmed to operate in a communications modein which it transfers information to an external optical signal detectorby modulation of the display. Communications may be with an externalcomputer through a boot which fits over the display during calibration.

The information stored in the EEPROM may include calibrationinformation. It may also establish a range and incremental response ofthe display to sensed radiation and other information whichcharacterizes the personality of a particular unit. For example, theinformation stored in the EEPROM may determine whether the display is indegrees Fahrenheit or degrees centigrade. That information may becontrolled by a switch to which the processor responds. The system mayinclude a sound source, and the stored information may determine thetiming at which the sound source is activated. For example, the storedinformation may cause the display to be locked to a reading apredetermined time after the radiation detector is turned on, and thestored information may cause the sound source to be activated when thedisplay is locked. Similarly, the stored information may cause theradiation detector to be turned off after a predetermined time and causethe sound source to be activated as the radiation detector is turnedoff. Alternatively, the stored information may cause the display toindicate the peak radiation sensed during a period of time and may causethe sound source to be activated when radiation sensed by the sensorapproximates the peak.

The information stored in the EEPROM may cause a conversion from sensedtympanic temperature to a temperature which approximates oral and/orcore temperature and which is displayed. The processor may also performconversions based on linear approximations, and the stored informationmay establish the end points and slopes of the linear approximations.For example, a linear approximation may be used to determine ambienttemperature from a thermistor output or to determine target temperaturefrom a thermistor output and a thermopile output.

The electronics support a simple watchdog operation associated with theon switch to the unit. An active device is turned on by the switch toapply power to the electronics. A capacitor stores, for a limited time,a charge which holds the active device after release of the switch. Theprocessor is programmed to periodically charge the capacitor after poweris applied through the active device. Failure of the processor to followa program routine results in discharge of the capacitor and turning offof the active device on the radiation detector. The switch may also becoupled directly to the processor so that the processor may respond toactuation of the switch after the radiation detector is turned on forother functions.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 illustrates a radiation detector for tympanic temperaturemeasurements in accordance with the present invention.

FIG. 2A is an illustration of a disposable sheet for covering a probe ofthe detector of FIG. 1; FIG. 2B is an illustration of a tape of thedisposable sheets of FIG. 2A; FIG. 2C is a perspective view of a cartoncontaining a stack of the sheets formed by a z fold of the tape of FIG.2B; and FIG. 2D is an illustration of a roll of such sheets.

FIG. 3A is a side illustration of the sheet of FIG. 2A pulled over theprobe of the radiation detector of FIG. 1; and FIG. 3B is a view of thesheet over the probe as viewed from line B--B of FIG. 3A.

FIG. 4 is a cross-sectional view of the extensions of the detector ofFIG. 1 in which the thermopile radiation sensor is positioned.

FIG. 5 is a cross-sectional view of the thermopile assembly of FIG. 4.

FIG. 6 is a block diagram of the electronic circuit of the detector ofFIG. 1.

FIG. 7 illustrates a boot positioned on the detector of FIG. 1 during acalibration procedure.

FIGS. 8A-8D are flow charts of the system firmware.

DESCRIPTION OF A PREFERRED EMBODIMENT

The radiation detector 12 of FIG. 1 includes a flat housing 14 with adigital display 16 for displaying a tympanic temperature measurement.Although the display may be located anywhere on the housing, it ispreferred that it be positioned on the end so the user is not inclinedto watch it during a measurement. The instrument makes an accuratemeasurement when rotated to scan the ear canal, and the user shouldconcentrate on only the scanning motion. Then the display can be read. Athermopile radiation sensor is supported within a probe 18 at theopposite end of the housing 14. The extension 18 extends orthogonallyfrom an intermediate extension 20 which extends at an angle of about 15degrees from the housing 14. As such, the head of the detector,including the extension 18 and 20, has the appearance of a conventionalotoscope. An on/off switch 22 is positioned on the housing.

A cross-sectional view of the extension of the detector is illustratedin FIG. 4. A base portion 23 is positioned within the housing 14, andthe housing clamps about a groove 24. As noted, the portion 20 extendsat about a 15 degree angle from the housing and thus from the baseportion 22. The extension 18 is tapered toward its distal end at 26 sothat it may be comfortably positioned in the ear to view the tympanicmembrane and/or ear canal.

A preferred disposable element to be used over the extension 18 isillustrated in FIG. 2A. It is a flat sheet 42 of one-half milstretchable plastic such as polyethylene which is transparent toinfrared. Reinforcement sheets 44 and 46 are provided at each end of theplastic sheet, and holes 48 and 50 are provided in the reinforcedregions. The flat sheet may be stretched over the distal end of theextension 18 and pressed over retainers such as pins 52 and 54protruding from the sides of the extension 18 as illustrated in FIGS. 3Aand 3B. Alternatively, a material to which polyethylene adheres may beprovided on the probe to retain the sheet. Although the flat sheet doesnot provide a close fit to the sides over the full length of theextension 18, it is sufficiently stretchable to form a neat fit at theend of the extension and is sufficiently flexible that it bends andcauses no discomfort to the patient when the extension is seated in theear.

The reinforcement sheets 44 and 46 serve as tabs which extend beyond theprobe. Those tabs can be readily grasped for positioning the sheet onthe probe and removing the sheet from the probe. Although the disposablecover could be formed without the reinforcement sheets, the stifferreinforcement sheets make the disposable cover much easier to handle.

The diameter of the end of the probe is about 3/8 inch, and the sheet 42is about two inches wide so that it folds over the probe end whenstretched. The distance between the holes is about 41/2 inches and thatdistance requires about 1/8 inch stretching of the sheet to secure it onthe probe.

The edge at the end of the probe is rounded so that when the probe isinserted into the ear it can be rotated somewhat without discomfort tothe patient. The probe is also curved like an otoscope to avoidinterference with the ear. By thus rotating the probe, the ear canal isscanned and, at some orientation of the probe during that scan, one canbe assured that the maximum temperature is viewed. Since the ear canalcavity leading to the tympanic area is the area of highest temperature,the instrument is set in a peak detection mode, and the peak detectedduring the scan is taken as the tympanic temperature.

An infrared reading of tympanic temperature, as opposed to an electronicthermometer reading of oral temperature, allows for a very inexpensivedisposable. The disposable need not be sufficiently rugged to preventcutting by teeth and the resultant contamination as is the case with anoral thermometer.

Preferably, individual sheets are torn from a tape of sheets illustratedin FIG. 2B. Sheets are formed from a continuous tape of polyethylene.Adhesive tape is placed periodically along the tape to providereinforcements. The tape is stamped to provide the levels at the ends ofeach sheet and to provide the holes 48 and 50. The reinforcement tapeand polyethylene may be perforated at 55 to facilitate tearing ofindividual sheets from the tape.

As illustrated in FIG. 2C, the tape may be folded in a z-fold fashion toform a stack of the disposable sheets packaged in a carton 57. As eachcover is torn off, another appears. Alternatively, the tape may beprovided in a roll as illustrated in FIG. 2D. The roll may also beplaced in a carton. In either case, the carton may be provided withadhesive so that it can be mounted to the side of the housing 14 in anapproach like that used with electronic thermometers.

As illustrated in FIG. 4, a thermopile 28 is positioned within a can 30to view the infrared radiation in the ear canal through a tube 32. Boththe base can 30 and the tube 32 are in close thermal contact with aconductive thermal mass including an annular member 34 and a plug 36 ofcopper. The outer sleeve 38 of the extension 18 and the intermediateextension 20 are of plastic material of low thermal conductivity. Thesleeve 38 is separated from the thermal mass 34 by an insulating airspace 40. The taper of the thermal mass 34 permits the insulating spaceto the end of the extension while minimizing the thermal resistance fromthe end of the tube 32 to the thermopile, a parameter discussed indetail below. The inner surface of the plastic sleeve 38 may be coatedwith a good thermal conductor to distribute across the entire sleeve anyheat received from contact with the ear. Twenty mils of copper coatingwould be suitable.

Details of the thermopile assembly within the can 30 are illustrated inFIG. 5. The tube 32 and can cylinder 30 are soldered together to form anintegral unit in which a gaseous medium of low thermal conductivity suchas xenon fills the tube and cylinder 30 and surrounds a thermopile flake28. The tube is closed by a germanium window 57. The interior of thetube is plated with gold to improve its reflectance to better guideradiation to the flake.

An alternative to forming the tube 32 and can 30 as a single unit wouldbe to use a conventional thermopile assembly with a window on the canand to position the waveguide tube in front of the can. However, toprevent contamination of the inner walls of the tube, a rigid windowshould be placed at the distal end of the tube. Two windows woulddiminish the radiation signal received by the thermopile.

The thermopile is mounted to the rear surface of a polyester sheet 58(sold under the trademark Mylar) supported on the rear surface of aberyllium oxide ring 60. Contact to the thermopile is made through pins62 and 64 which extend through a stack of beryllium oxide rings 66.Beryllium oxide is used because it is an electrical insulator yet a goodthermal conductor. A case conductor 68 contacts the casing. Connectionto the thermopile is by a conductive film printed on the sheet 58. Toprevent abrasion of that film, the ring 60 is usually spaced slightlyfrom the adjacent ring 66. As noted below, good thermal conduction isimportant in implementing the present invention, and it was found thatthe xenon gas filling the gap significantly reduced the conduction tothe cold junction of the thermopile. In accordance with one feature ofthe present invention, that space 70 is filled with a conductivematerial. It is preferable that the material be filled with an epoxy ofgood thermal conductivity, but virtually any material offers asubstantial improvement over the lack of conduction through the xenon.

One of the design goals of the device was that it always be in propercalibration without requiring a warm-up time. This precluded the use aheated target in a chopper unit or heating of the cold junction of thethermopile as was suggested in the above-mentioned O'Hara et.al. patent.To accomplish this design goal, it is necessary that the system be ableto operate with the thermopile at any of a wide range of ambienttemperatures and that the thermopile output have very low sensitivity toany thermal perturbations.

The output of the thermopile is a function of the difference intemperature between its warm junction, heated by radiation, and its coldjunction which is in close thermal contact with the can 30. In orderthat the hot junction respond only to radiation viewed through thewindow 57, it is important that the tube 32 be, throughout ameasurement, at the same temperature as the cold junction. To that end,changes in temperature in the tube 32 must be held to a minimum, and anysuch changes should be distributed rapidly to the cold junction to avoidany thermal gradients. To minimize temperature changes, the tube 32 andthe can 30 are, of course, well insulated by means of the volume of air40. Further, a high conductance thermal path is provided to the coldjunction. The tube 32 and can 30 are in close thermal communication withthe thermal masses 34 and 36, and the high conductivity and thickness ofthe thermal masses increase the thermal conductance. A high thermalconductivity epoxy, solder or the like joins the tube, can and thermalmasses. The solder or epoxy provides a significant reduction in thermalresistance. Where solder is used, to avoid damage to the thermopilewhich is rated to temperatures of 125° C., a low temperature solder ofindium-tin alloy which flows at 100° C. is allowed to flow into theannular mass 34 to provide good thermal coupling between all elements.

The thermal resistance from the outer surface of the plastic sleeve 38to the conductive thermal mass is high to minimize thermal perturbationsto the inner thermal mass. To minimize changes in temperature of thetube 32 with any heat transfer to the tube which does occur, the thermalmass of the tube 32, can 30, annular mass 34 and plug 36 should belarge. To minimize thermal gradients where there is some temperaturechange in the tube during measurement, the thermal resistance betweenany two points of the thermal mass should be low.

Thus, due to the large time constant of the thermal barrier, anyexternal thermal disturbances, such as when the extension contacts skin,only reach the conductive thermal mass at extremely low levels during ameasurement period of a few seconds; due to the large thermal mass ofthe material in contact with the cold junction, any such heat transferonly causes small changes in temperature; and due to the good thermalconductance throughout the thermal mass, any changes in temperature aredistributed quickly and are reflected in the cold junction temperaturequickly so that they do not affect temperature readings.

The thermal RC time constant for thermal conduction through the thermalbarrier to the thermal mass and tube should be at least two orders ofmagnitude greater than the thermal RC time constant for the temperatureresponse of the cold junction to heat transferred to the tube andthermal mass. The RC time constant for conduction through the thermalbarrier is made large by the large thermal resistance through thethermal barrier and by the large thermal capacitance of the thermalmass. The RC time constant for response of the cold junction is made lowby the low resistance path to the cold junction through the highlyconductive copper tube, can and thermal mass, and the low thermalcapacitance of the stack of beryllium oxide rings and pin conductors tothe thermopile.

Although the cold junction capacitance is naturally low, there are sizeconstraints in optimizing the thermal capacitance of the thermal mass,the thermal resistance through the thermal barrier and the internalthermal resistance. Specifically, the external thermal resistance can beincreased by increased radial dimensions, the capacitance of the thermalmass can be increased by increasing its size, and the thermal resistancethrough the longitudinal thermal path through the tube can be decreasedby increasing its size. On the other hand, the size must be limited topermit the extension to be readily positioned and manipulated within theear.

Besides the transfer of heat from the environment, another significantheat flow path to the conductive thermal mass is through leads to thesystem. To minimize heat transfer through that path, the leads are keptto small diameters. Further, they are embedded in the plug 36 throughbores 70; thus, any heat brought into the system through those leads isquickly distributed throughout the thermal mass, and only small changesin temperature and small gradients result.

Because the temperature of the thermal mass is not controlled, and theresponse of the thermopile 28 is a function of its cold junctiontemperature, the cold junction temperature must be monitored. To thatend, a thermistor is positioned at the end of a central bore 72 in theplug 36.

A schematic illustration of the electronics in the housing 14, forproviding a temperature readout on display 16 in response to the signalfrom the thermopile, is presented in FIG. 6. The system is based on amicroprocessor 73 which processes software routines included in readonly memory within the processor chip. The processor may be a 6805processor sold by Motorola.

The voltage generated across the thermopile 28 due to a temperaturedifferential between the hot and cold junctions is amplified in anoperational amplifier 74. The analog output from the amplifier 74 isapplied as one input to a multiplexer 76. Another input to themultiplexer 76 is a voltage taken from a voltage divider R1, R2 which isindicative of the potential V. from the power supply 78. A third inputto the multiplexer 76 is the potential across a thermistor RT1 mountedin the bore 72 of block 36. The thermistor RT1 is coupled in a voltagedivider circuit with R3 across a reference potential VRef. The finalinput to the multiplexer is a potential taken from a potentiometer R4which may be adjusted by a user. The system may be programmed to respondto that input in any of a number of ways. In particular, thepotentiometer may be used as a gain control or as a DC offset control.

At any time during the software routine of the microprocessor 73, one ofthe four inputs may be selected by the select lines 78. The selectedanalog signal is applied to a multiple slope analog system 80 used bythe microprocessor in an integrating analog-to-digital conversion 80.The subsystem 80 may be a TSC500A sold by Teledyne. It utilizes thereference voltage VRef from a reference source 82. The microprocessor 73responds to the output from the convertor 80 to generate a countindicative of the analog input to the convertor.

The microprocessor drives four 7-segment LED displays 82 in amultiplexed fashion. Individual displays are selected sequentiallythrough a column driver 84, and within each selected display the sevensegments are controlled through segment drivers 86.

When the switch 22 on the housing is pressed, it closes the circuit fromthe battery 78 through resistors R5 and R6 and diode D1 to ground. Thecapacitor C1 is quickly charged, and field effect transistor T1 isturned on. Through transistor T1, the V+ potential from the storage cell78 is applied to a voltage regulator 86. The regulator 86 provides theregulated +5 volts to the system. It also provides a reset signal to themicroprocessor. The reset signal is low until the +5 volt reference isavailable and thus holds the microprocessor in a reset state. When the+5 volts is available, the reset signal goes high, and themicroprocessor begins its programmed routine.

When the switch 22 is released, it opens its circuit, but a charge ismaintained on capacitor C1 to keep transistor T1 on. Thus, the systemcontinues to operate. However, the capacitor C1 and transistor T1provide a very simple watchdog circuit. Periodically, the microprocessorapplies a signal through driver 84 to the capacitor C1 to recharge thecapacitor and thus keep the transistor T1 on. If the microprocessorshould fail to continue its programmed routine, it fails to charge thecapacitor C1 within a predetermined time during which the charge on C1leaks to a level at which transistor T1 turns off. Thus, themicroprocessor must continue in its programmed routine or the systemshuts down. This prevents spurious readings when the processor is notoperating properly.

With transistor T1 on, the switch 22 can be used as an input throughdiode D2 to the microprocessor to initiate any programmed action of theprocessor.

In addition to the display, the system has a sound output 90 which isdriven through the driver 84 by the microprocessor.

In order to provide an analog output from the detector, adigital-to-analog convertor 92 is provided. When selected by line 94,the convertor converts serial data on line 96 to an analog output madeavailable to a user.

In accordance with one aspect of the present invention, both calibrationand characterization data required for processing by the microprocessormay be stored in an electrically erasable programmable read only memory(EEPROM) 100. The EEPROM may, for example, be a 93c46 sold byInternational CMOS Technologies, Inc. The data may be stored in theEEPROM by the microprocessor when the EEPROM is selected by line 102.Once stored in the EEPROM, the data is retained even after power down.Thus, though electrically programmable, once programmed the EEPROMserves as a virtually nonvolatile memory.

Prior to shipment, the EEPROM may be programmed through themicroprocessor to store calibration data for calibrating the thermistorand thermopile. Further, characterization data which defines thepersonality of the infrared detector may be stored. For example, thesame electronics hardware, including the microprocessor 73 and itsinternal program, may be used for a tympanic temperature detector inwhich the output is accurate in the target temperature range of about60° F. to a 110° F. or it may be used as an industrial detector in whichthe target temperature range would be from about 0° F. to 100° F.Further, different modes of operation may be programmed into the system.For example, several different uses of the sound source 90 areavailable.

Proper calibration of the detector is readily determined and the EEPROMis readily programmed by means of an optical communication link whichincludes a transistor T2 associated with the display. As illustrated inFIG. 7, a communication boot 104 may be placed over the end of thedetector during a calibration/characterization procedure. A photodiodein the boot generates a digitally encoded optical signal which isfiltered and applied to the detector T2 to provide an input to themicroprocessor 73. In a reverse direction, the microprocessor, maycommunicate optically to a detector in the boot by flashing specificsegments of the digital display 82. Through that communication link, anoutside computer 106 can monitor the outputs from the thermistor andthermopile and perform a calibration of the devices. A unit to becalibrated is pointed at each of two black body radiation sources whilethe microprocessor 73 converts the signals and sends the values to theexternal computer. The computer is provided with the actual black bodytemperatures and ambient temperature in the controlled environment ofthe detector, computes calibrator variables and returns those variablesto be stored in the detector EEPROM. Similarly, data which characterizesa particular radiation detector may be communicated to themicroprocessor for storage in the EEPROM.

A switch 108 is positioned behind a hole 110 (FIG. 1) in the radiationdetector so that it may be actuated by a rigid metal wire or pin.Through that switch, the user may control some specific mode ofoperation such as converting the detector from degrees Fahrenheit todegrees centigrade. That mode of operation may be stored by themicroprocessor 73 in the EEPROM so that the detector continues tooperate in a specific mode until a change is indicated by closing theswitch 108.

A switch 106 may be provided either internally or through the housing tothe user to set a mode of operation of the detector. By positioning theswitch at either the lock position, the scan position or a neutralposition, any of three modes may be selected. The first mode is thenormal scan mode where the display is updated continuously. A secondmode is a lock mode where the display locks after a selectable delay andthen remains frozen until power is cycled or, optionally, the power-onbutton is pushed. The sound source may be caused to sound at the time oflock. The third mode is the peak mode where the display reads themaximum value found since power-on until power is cycled or, optionally,the power-on button is pushed.

The processor determines where the voltage from the divider R1, R2 dropsbelow each of two thresholds. Below the higher threshold, the processorperiodically enables the sound source to indicate that the battery islow and should be replaced but allows continued readout from thedisplay. Below the lower threshold, the processor determines that anyoutput would be unreliable and no longer displays temperature readings.The unit would then shut down upon release of the power button.

To provide a temperature readout, the microprocessor makes the followingcomputations: First the signal from thermistor RT1 is converted totemperature using a linear approximation. Temperature is defined by aset of linear equations

    y=M(x-xo)+b

where x is an input and xo is an input end point of a straight lineapproximation. The values of M, xo and b are stored in the EEPROM aftercalibration. Thus, to obtain a temperature reading from the thermistor,the microprocessor determines from the values of xo the line segment inwhich the temperature falls and then performs the computation for ybased on the variables M and b stored in the EEPROM.

A fourth power representation of the ambient temperature is thenobtained by a lookup table in the processor ROM. The sensed radiationmay be corrected using a calibration factor, a sensor gain temperaturecoefficient, the detected ambient temperature and a calibrationtemperature stored in the EEPROM. The corrected radiation signal and thefourth power of the ambient temperature are summed, and the fourth rootis taken. The fourth root calculation is also based on a linearapproximation which is selected according to the temperature range ofinterest for a particular unit. Again, the break points and coefficientsfor each linear approximation are stored in the EEPROM and are selectedas required. To the thus computed target temperature is added anadjustment factor which may, for example, allow for a reading whichclosely corresponds to oral and/or core temperature based on theknowledge of the relationship of oral and/or core temperature totympanic temperature. Also added to the calculated temperature is a usertweak obtained from resistor R4.

An additional factor based on ambient temperature may also be includedas an adjustment. The temperature of the ear T_(e) which is sensed bythe thermopile is not actually the core temperature T_(c). There isthermal resistance between T_(c) and T_(e). Further, there is thermalresistance between the sensed ear temperature and the ambienttemperature. The result is a sense temperature T_(e) which is a functionof the core temperature of interest and the ambient temperature. Basedon an assumed constant c which is a measure of the thermal resistancesbetween T_(c), T_(e) and T_(a), core temperature can be computed as##EQU1## This computation can account for a difference of from one-halfto one degree between core temperature and sensed ear temperature,depending on ambient temperature.

The actual computations performed by the processor are as follows,where:

H is the radiation sensor signal

Hc is corrected H (deg K⁴)

Tamb is ambient temperature (deg F)

Taf is 4th power of Tamb (deg K⁴)

Tt is target temperature (deg F)

Tz is ambient temp during cal (deg F)

Td is the displayed temperature

Rt is the thermistor signal

Kh is a radiation sensor gain cal factor

Zt is a thermistor zero cal factor

Kt is a sensor gain temperature coefficient (%/deg F)

s is the Stefan-Boltzmann constant

F is an adjustment factor

Ut is a user tweak

Tamb(deg F)=Thermistor lookup table (Rt)-Zt

Hc(deg K⁴)=Kh * H * (1+Kt * (Tamb-Tz))/s

Taf(deg K⁴)=4th power lookup table (Tamb)

Tt(deg F)=(Hc+Taf)^(1/4) (Final lookup table)

Tt(deg C)=(5/9) * (Tf(deg F)-32) optional

Td=Tt+F+Ut

The following is a list of the information which may be contained in theEEPROM and therefore be programmable at the time of calibration:

Radiation sensor offset

Radiation sensor gain

Radiation sensor temperature coefficient

Thermistor offset

Ambient temperature at calibration

Thermistor lookup table

Final temperature lookup table

Adjustment factor F

Sound source functions:

Beep at button push in lock mode

none/20/40/80 milliseconds long

Beep at lock

none/20/40/80 milliseconds long

Beep at power down

none/20/40/80 milliseconds long

Beep at lowbattery

none/20/40/80 milliseconds long

interval 1/2/3 sec

single/double beep

Timeout functions:

Time to power-down

0.5 to 128 sec in 0.5 sec increments

Delay until lock

0.5 to 128 sec in 0.5 sec increments

Other functions:

Power-on button resets lock cycle

Power-on button resets peak detect

Display degrees C / degrees F

EEPROM "Calibrated" pattern to indicate that the device has beencalibrated

EEPROM checksum for a self-check by the processor

FIGS. 8A-8D provide a flowchart of the firmware stored in themicroprocessor 73. From reset when the instrument is turned on, thesystem is initialized at 110 and the contents of the EEPROM are readinto memory in the microprocessor at 112. At 114, the processor readsthe state of power and mode switches in the system. At 116, the systemdetermines whether a mode switch 113 has placed the system in aself-test mode. If not, all eights are displayed on the four-digitdisplay 82 for a brief time. At 120, the system performs all A-to-Dconversions to obtain digital representations of the thermopile outputand the potentiometer settings through multiplexor 76.

The system then enters a loop in which outputs dictated by the modeswitch are maintained. First the timers are updated at 122 and theswitches are again read at 124. When the power is switched off, from 126the system enters a power down loop at 128 until the system is fullydown. At 130, the mode switch is checked and if changed the system isreset. Although not in the tympanic temperature detector, some detectorshave a mode switch available to the user so that the mode of operationcan be changed within a loop.

At 132, 136 and 140, the system determines its mode of operation andenters the appropriate scan process 134, lock process 138 or Peakprocess 142. In a scan process, the system updates the output to thecurrent reading in each loop. In a lock process, the system updates theoutput but locks onto an output after some period of time. In the peakprocess, the system output is the highest indication noted during ascan. In each of these processes, the system may respond to theprogramming from the EEPROM to perform any number of functions asdiscussed above. In the peak process which is selected for the tympanictemperature measurement, the system locks onto a peak measurement aftera preset period of time. During assembly, the system may be set at atest mode 144 which will be described with respect to FIG. 8D.

In any of the above-mentioned modes, an output is calculated at 146.Then the system loops back to step 122. The calculation 146 isillustrated in FIG. 8B.

At 148 in FIG. 8B, the raw sensor data is obtained from memory. Thesensor offset taken from the EEPROM is subtracted at 150, end theambient temperature previously obtained from the potentiometer RT1 isaccessed at 152. The temperature coefficient adjustment is calculated at154. At I56, the sensed signal is multiplied by the gain from EEPROM andby the temperature coefficient. At 158, the fourth power of the ambienttemperature is obtained, and at 160 it is added to the sensor signal. At162, the fourth root of the sum is obtained through a lookup table.Whether the display is in degrees centigrade or degrees Fahrenheit isdetermined at 164. If in degrees centigrade, a conversion is performedat 166. At 168, adjustment values, including that from the potentiometerR4, are added.

Analog-to-Digital conversion is performed periodically during aninterrupt to the loop of FIG. 8A which occurs every two milliseconds.The interrupt routine is illustrated in FIG. 8C. Timer counters areupdated at 170. A-to-D conversions are made from 172 only every 100milliseconds when a flag has been set in the prior interrupt cycle.During most interrupts, an A/D conversion does not occur. Then, the100-millisecond counter is checked at 174, and if the count has expired,a flag is set at 176 for the next interrupt. The flag is checked at 178and, if found, the display is updated at 180. The system then returns tothe main loop of FIG. 8A.

Where the 100 millisecond flag is noted at 172, an A-to-D conversion isto be performed. The system first determines at 182 whether a countindicates there should be a conversion of the thermopile output at 184or a conversion of the thermistor output at 186. The thermopile sensorconversion is performed nine out of ten cycles through the conversionloop. At 188, the system checks to determine whether a conversion ismade from the potentiometer R4 or from the battery voltage divider R1,R2 at 192. These conversions are made alternately.

FIG. 8D illustrates the self-test sequence which is called by the modeswitch 113 only during assembly. During the test, the beeper sounds at182 and all display segments are displayed at 184. Then the system stepseach character of the display from zero through nine at 186. The systemthen enters a test loop. At 188, the system senses whether the button108 has been pressed. If so, a display counter is incremented at 190.The display for the unit then depends on the count of the displaycounter. With the zero count, the adjustment potentiometer value isdisplayed at 192. Thereafter, if the display counter is incremented bypressing the button 108, the raw sensor data is displayed. With the nextincrement, ambient temperature is displayed at 196, and with the nextincrement, the raw output from the ambient temperature sensor RT1 isdisplayed. With the next increment, the battery voltage is displayed.After the test, the assembler sets the mode switch to the properoperating mode.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

We claim:
 1. A radiation detector comprising:a thermopile mounted withina thermal mass and having a junction thermally coupled to the thermalmass; a thermally conductive tube coupled to the thermal mass forguiding radiation to the thermopile from an external target; and athermal barrier surrounding the thermal mass and tube; the outer thermalRC time constant for thermal conduction through the thermal barrier tothe thermal mass and tube is at least two orders of magnitude greaterthan the inner thermal RC time constant for the temperature response ofthe cold junction to heat transferred to the tube and thermal massthrough the thermal barrier.
 2. A radiation detector as claimed in claim1 wherein the outer RC time constant is at least three orders ofmagnitude greater than the inner RC time constant.
 3. A radiationdetector as claimed in claim 1 wherein the inner RC time constant isabout 1/2 second or less.
 4. A radiation detector as claimed in claim 1comprising a continuous low thermal resistance path from the end of thetube to the junction of the thermopile.
 5. A radiation detector asclaimed in claim 1 wherein the thermopile is mounted within a lowconductivity gaseous environment which extends through the length of theconductive tube.
 6. A radiation detector as claimed in claim 1 whereinthe thermopile is mounted to a film suspended within a ring, the ringbeing supported on electrically conductive pins extending through anadjacent ring to the side of the film on which the thermopile ismounted, the film being spaced from the adjacent ring, the rings andthermopile being surrounded by a low conductivity gaseous volume, theradiation detector further comprising the improvement wherein the spacebetween the film and the adjacent ring through which the conductorsextend is filled with thermally conductive material.
 7. A radiationdetector as claimed in claim 1 wherein:the thermopile is mounted withina low conductivity gaseous volume within a can; the thermal masscomprises an annular member which surrounds the can and a length of thetube adjacent to the can, the annular member being tapered about itsouter periphery toward the tube, and a conductive plug positioned behindthe can within the annular member, the can, tube, annular member andplug being bonded together by high thermal conductivity material; andthe thermal barrier comprises a sleeve spaced from the thermal mass andtube, the sleeve being tapered toward the end of the tube away from thecan.
 8. A radiation detector as claimed in claim 1 adapted to provide anindication of tympanic temperature.
 9. In a thermopile assemblycomprising a thermopile mounted to a film suspended within a ring, thering being supported on electrically conductive pins extending throughan adjacent ring to the side of the film on which the thermopile ismounted, the film being spaced from the adjacent ring, and the rings andthermopile being surrounded by a low conductivity gaseous volume, theimprovement wherein the space between the film and the adjacent ringthrough which the conductors extend is filled with thermally conductivematerial.
 10. A radiation detector comprising a thermopile mountedwithin a can and a waveguide tube of lesser internal diameter than thecan and integral with the can, the tube having an airtight window at adistal end thereof and directing radiation which passes through thewindow to the thermopile, a gaseous environment having a thermalconductivity significantly lower than that of air being maintained aboutthe thermopile within the can and through the length of the tube.
 11. Atympanic temperature detector comprising:a housing adapted to be held byhand; an extension from the housing adapted to be inserted into an ear,the extension supporting a radiation sensor and having a window at theend thereof through which the sensor receives radiation from a tympanicmembrane area; a temperature display on the housing for displayingtympanic temperature; battery powered electronics in the housing forconverting radiation sensed by the sensor to temperature displayed bythe display; and a removable plastic sheet stretched over the end of theextension, the sheet having holes at opposite ends thereof which aresecured to retaining members on the side of the extension to retain thesheet on the extension.
 12. A tympanic temperature detector as claimedin claim 11 wherein the radiation sensor is a thermopile, the coldjunction of which is allowed to follow ambient temperature.
 13. Atympanic temperature detector as claimed in claim 12 wherein theradiation sensor is a thermopile mounted within a thermal mass in theextension and having a junction thermally coupled to the thermal mass, athermally conductive tube is coupled to the thermal mass for guidingradiation to the thermopile from an external target and a thermalbarrier surrounds the thermal mass and tube, the outer thermal RC timeconstant for thermal conduction through the thermal barrier to thethermal mass and tube is at least two orders of magnitude greater thanthe inner thermal RC time constant for the temperature response of thecold junction to heat transferred to the tube and thermal mass throughthe thermal barrier.
 14. A tympanic temperature detector as claimed inclaim 13 wherein the inner thermal RC time constant is about 1/2 secondor less.
 15. A tympanic temperature detector as claimed in claim 12further comprising an airtight waveguide tube mounted in the extensionintegral with a can containing the thermopile, the tube guidingradiation from the window to the thermopile and being filled with a lowconductivity gas.
 16. A tympanic temperature detector as claimed inclaim 12 wherein the thermopile is mounted to a film suspended within aring, the ring being supported on electrically conductive pins extendingthrough an adjacent ring to the side of the film on which the thermopileis mounted, the film being spaced from the adjacent ring, the rings andthermopile being surrounded by a low conductivity gaseous volume, theradiation detector further comprising the improvement wherein the spacebetween the film and the adjacent ring through which the conductorsextend is filled with thermally conductive material.
 17. A tympanictemperature detector as claimed in claim 12 wherein:the thermopile ismounted within a low conductivity gaseous volume within a can; thethermal mass comprises an annular member which surrounds the can and alength of the tube adjacent to the can, the annular member being taperedabout its outer periphery toward the tube, and a conductive plugpositioned behind the can Within the annular member, the can, tube,annular member and plug being bonded together by high thermalconductivity material; and the thermal barrier comprises a sleeve spacedfrom the thermal mass and tube, the sleeve being tapered toward the endof the tube away from the can.
 18. A tympanic temperature detector asclaimed in claim 11 which provides a display of tympanic temperaturewithin five seconds of inserting the extension into the ear.
 19. Atympanic temperature detector as claimed in claim 11 wherein theradiation sensor is a thermopile and the window is positioned at the endof a waveguide tube integral with a can surrounding the thermopile, alow conductivity gaseous environment surrounding the thermopile withinthe can and extending through the length of the conductive tube.
 20. Atympanic temperature detector as claimed in claim 11 further comprisinga tape of said removable plastic sheets, individual sheets being adaptedto be torn from the tape to be stretched over the end of the extension.21. A tympanic temperature detector as claimed in claim 11 wherein theextension extends about orthogonally from an intermediate extensionwhich extends at an angle of about 15 degrees from an end of thehousing, the extension being curved outwardly along its length from itsdistal end.
 22. A tympanic temperature detector as claimed in claim 11further comprising a processor for providing the temperature displayedon the housing as a function of the received radiation compensated by anindication of ambient temperature to provide a core temperatureapproximation.
 23. A tympanic temperature detector comprising:a housingadapted to be held by hand; an extension from the housing adapted to beinserted into an ear, the extension supporting a radiation sensor andhaving a window at the end thereof through which the sensor receivesradiation from a tympanic membrane area; a temperature display on thehousing for displaying tympanic temperature; battery powered electronicsin the housing for converting radiation sensed by the sensor totemperature displayed by the display, the electronics including aprocessor for providing the temperature displayed on the housing as afunction of the received radiation compensated by an indication ofambient temperature to provide a core temperature approximation.
 24. Atympanic temperature detector comprising:a housing adapted to be held byhand; an extension from the housing adapted to be inserted into an ear,the extension supporting a thermopile radiation sensor and having awindow at the end thereof through which the sensor receives radiationfrom a tympanic membrane area; a temperature display on the housing fordisplaying tympanic temperature; battery powered electronics in thehousing for converting radiation sensed by the sensor to temperaturedisplayed by the display; a removable plastic sheet stretched over theend of the extension; and a thermal mass in the extension within whichthe thermopile is mounted, the thermopile having a junction thermallycoupled to the thermal mass, and therein a thermally conductive tube iscoupled to the thermal mass for guiding radiation to the thermopile froman external target, a thermal barrier surrounds the thermal mass andtube, and the outer thermal RC time constant for thermal conductionthrough the thermal barrier to the thermal mass and tube is at least twoorders of magnitude greater than the inner thermal RC time constant forthe temperature response of the cold junction to heat transferred to thetube and thermal mass through the thermal barrier.
 25. A tympanictemperature detector as claimed in claim 24 wherein the inner thermal RCtime constant is about 1/2 second or less.
 26. A tympanic temperaturedetector comprising:a housing adapted to be held by hand; an extensionfrom the housing adapted to be inserted into an ear, the extensionsupporting a radiation sensor and having a window at the end thereofthrough which the sensor receives radiation from a tympanic membranearea, the radiation sensor being a thermopile having a cold junctionwhich is allowed to follow ambient temperature, the thermopile beingmounted to a film suspended within a ring which is supported onelectrically conductive pins extending through an adjacent ring to theside of the film on which the thermopile is mounted, the film beingspaced from the adjacent ring and the rings and thermopile beingsurrounded by a low conductivity gaseous volume, the radiation detectorfurther comprising the improvement wherein the space between the filmand the adjacent ring through which the conductive pins extend is filledwith thermally conductive material; a temperature display on the housingfor displaying tympanic temperature; and battery powered electronics inthe housing for converting radiation sensed by the sensor to temperaturedisplayed by the display.
 27. A tympanic temperature detectorcomprising:a housing adapted to be held by a hand; an extension from thehousing adapted to be inserted into an ear, the extension supporting aradiation sensor and having a window at the end thereof through whichthe sensor receives radiation from a tympanic membrane area, and whereinthe extension extends about orthogonally from an intermediate extensionwhich extends at an angle of about 15 from an end of the housing, theextension being curved outwardly along its length from its distal end; atemperature display on the housing displaying tympanic temperatures; andbattery powered electronics in the housing for converting radiationsensed by the sensor to temperature displayed by the display.
 28. Aradiation detector comprising:a thermopile mounted within a thermal massand having a junction thermally coupled to the thermal mass; a thermallyconductive tube coupled to the thermal mass for guiding radiation to thethermopile from an external target; and a thermal barrier surroundingthe thermal mass and tube; an inner thermal RC time constant for thetemperature response of the cold junction to heat transferred to thetube and thermal mass through the thermal barrier along being about 1/2second or less.